Hey there, I've been reading a lot about piezoelectric materials

Hey there, I've been reading a lot about piezoelectric materials lately and I've steel looking for an answer for a major question -
Is there any way to calculate the watt output of a certain piezoelectric component* ?

*The piezoelectric components I'm talking about are those which convert mechnical force to electricite ( for example , tiny piezoelectric components which make a led turn on once they're touched , like this one : http://www.youtube.com/watch?v=Xuw9frP1GNo